摘要：Nodular cast iron is material often used in casting processes after gray iron. FCD 55 is nodular cast iron having ultimate strength ±55 kgf/mm2. Its mechanical properties, i.e. strength, toughness, and wear resista e, are able to increase by austempering process. The nodular cast iron which has een austempered is called ADI. The aim of this research is to obtain the optimum austempering temperature and time to improve the mechanical properties of the FCD55, especially its hardness. The FCD 55 material used to make the testing specimens is elted using an electrical induction furnace and then pouring into the “Y Block” moulds. “Y Block” casting specimens are machined to make testing specimens. The pecimen testing involves hardness testing and microstructural investigation (metallography). The hardness testing dan microstructural investigation is performed for specimens that have not been austempered dan specimens which have been austempered. The austempering process is performed at austenitizing temperature 900oC, for 1 h meanwhile the austempering process parameters, i.e. the temperature s varied 300oC, 350oC and 400oC and the time is varied for 30, 60 and 90 minutes. T e number of specimens for each testing are 5 for specimens which have not been austempered dan 3 for each parameter variation of the austempering process.. The ustenitizing process is performed using an electrical resistance furnace and the austempering rocess using cooling media consist of 50% NaNO3 and 50% KNO3. From the microstructural investigation, the higher austempering temperature for the same austempering time, the coarser ausferrite formed. This is due to the retained austenite which is large so that the acicular ferrite formed is smaller. The longer the austempering time, the larger the retained austenite content although the increase is not significant. From the hardness testing, it is able to conclude that the austempering time and temperature affect the hardness of the ADI. The longer and the hig r the austemperinng process the lower the hardness of material resulted.
